Water In Silicate Melts Research Articles

Ion microprobe studies of water in silicate melts: Temperature-dependent water diffusion in obsidian

The temperature dependence of water diffusivity in rhyolite melts over the range 650–950°C and [ P T(H 2O] = 700 bars is evaluated from water concentration-distance profiles measured in glass with an ion microprobe. Diffusivities are exponentially dependent on concentration over this temperature range and vary from about 10 −8 cm 2/s at 650°C to about 10 −7 cm 2/s at 950°C at 2 wt.% water. Water solubility also varies with temperature at a rate of −0.14 wt. per 100°C increase. The avtivation energy ( E a) appears to be constant at 19 ± 1kal/mole for 1, 2,and 3 wt.% H 2O. Comparison of these data with results for cation diffusion indicates that this value is a minimum E a for diffusion of any species in a rhyolite melt. Compensation plots of log 10 D 0 (the frequency factor) versus E a indicate that hydrous rhyolite melts follow the same trend as anhydrous basalts. D 0 increases for H 2O and Ca 2+ [1] as E a decreases. This suggests that these molecules may diffuse by different mechanisms than do monovalent cations, and that hydration of the melt affects diffusion of Ca 2+ and H 2O differently than it does monovalent cation diffusion. The results imply that dramatic increases in cation diffusivities by hydration [1] may occur with additions of less than 1 wt.% H 2O.

Ion microprobe studies of water in silicate melts. Concentration-dependent water diffusion in obsidian

The ion microprobe at Johnson Space Center has been calibrated for in situ water determinations on a 10-μm scale over the range 0.2 wt.% H 2O to 1.8, 6.8, and 3.7 wt.%, for basaltic, albitic, and rhyolitic glasses, respectively. The basalt glass calibration curve differs substantially from those of albite and rhyolite glasses, indicating a need to carefully match composition and/or melt structure between H 2O standards and unknowns. A value for the diffusivity of water as a function of concentration and time has been calculated from water diffusion profiles measured in rhyolite glasses prepared at 850°C and 700 bars P t(H 2O) [1]. Transient diffusion into a semi-infinite medium is described by the equation: −(φ/2)∂¸/∂φ=∂(D w∂¸/∂φ)/∂φ #x003B8;=1, φ=0, θ→ 0, θ→∞ , where x =distance from the cylinder edge, t =time, C 0 =initial concentration , C s =concentration at the edge , C =concentration at x, θ = C − C 0/C s − C 0 , φ = x/t 1/2 , and D w =diffusivity of water . An iterative technique has been used to calculate solutions to the diffusion equation as a function of D w [2]. Comparison of these solutions with the ion probe data indicate that, for 0.2wt.% ≤ C ≤ 3.7wt.%H 2O , D w can be described by an exponential function of θ, of the form D w = D 0exp(bθ) , with D 0 (i.e., D w at 0.2%) = (0.8−2.2) × 10 −8 cm 2/s and 2 ≤ b ≤ 4.

A method for determining the solubility of water in silicate melts

The method described in this paper for determining the solubility of water in silicate melts consists primarily of locating the boundary (or projected boundary) between the field of liquid and the field of liquid+gas in successive isobaric T-X _H2O sections for a given system. It involves the sealing of known amounts of water and silicate starting material in noble-metal capsules, the fusing and homogenization of these charges in pressure vessels, equilibration at designated experimental run temperatures and pressures, and quenching after prescribed periods of time. The quenched products are then examined for evidences of excess water. In determining solubility values from the results of critical runs, corrections are made for aqueous bubbles initially trapped and subsequently preserved within the charges, i.e., for water effectively unavailable for solution in the melt. An alternate method involves locating, also in successive isobaric T-X _H2O sections for a given system, the boundary (or projected boundary) between the field of liquid and the field of liquid + crystals. This method generally yields results that are less satisfactory, owing chiefly to the moderate slopes of this boundary for many systems and to difficulties in obtaining the precise control and measurements of temperature that are required. The solubility of water in melts derived from albite glass and Harding, New Mexico, pegmatite have been determined by the excess-water method as a function of pressure up to 10,000 bars and at temperatures of the H ₂ O-saturated liquidus for each melt. Solubility values for the pegmatite melt are 4.2 wt % at 1000 bars and 655 degrees C., 6.6% at 2000 bars and 635 degrees C., 11.2% at 5000 bars and 650 degrees C., and nearly 20% at 10,000 bars and 675 degrees C. Corresponding values for albite melt are 4.2 wt % (930 degrees C.), 6.4% (850 degrees C.), 9.9% (750 degrees C.), and 16.8% (700 degrees C.). The temperature coefficient of water solubility in Harding pegmatite melt, at a pressure of 1000 bars, is about -0.2 wt % of water per 100 degrees C. rise in temperature through the range 650 degrees to 850 degrees C. The solubility values obtained in these investigations are substantially lower, at pressures up to about 4500 bars, than values reported by Goranson for comparable materials on the basis of his weight-loss-on-ignition method. However, they are much higher, at pressures above 5000 bars, than values derived by the most reasonable extrapolations of Goranson9s results to these higher pressures. The discrepancies are attributed mainly to aqueous bubbles that normally are trapped and preserved in the experimental charges; these bubbles evidently were reckoned as dissolved water in the earlier work on albite and granite melts, which would account for pressure-solubility curves of seemingly incorrect position and shape.